Work machines such as, for example, excavators, loaders, dozers, and other types of heavy machinery use multiple hydraulic actuators in conjunction with a linkage system to accomplish a variety of tasks. The hydraulic actuators may include a tube having a head-end pressure chamber and a rod-end pressure chamber separated by a piston assembly. The tube may be connected to one portion of the linkage assembly, while the piston assembly may be connected to a different portion. The head and rod-end pressure chambers may be selectively filled with or drained of pressurized fluid to move the piston assembly relative to the tube, which affects movement of the linkage system. During movement of the linkage system, it is possible for gravity acting on the linkage system to cause the piston assembly to force draining of fluid from one of the rod or head-end chambers faster than fluid can fill the other of the rod or head-end chambers. In this situation, a void or vacuum may be created by the expansion of the filling chamber (voiding). Voiding can result in undesired and/or unpredictable movement of the work machine and could damage the hydraulic actuators.
One method of minimizing voiding within a hydraulic actuator is described in U.S. Pat. No. 5,868,059 (the '059 patent) issued to Smith on Feb. 9, 1999. The '059 patent describes an electrohydraulic valve arrangement in combination with an implement pump, a tank, and a hydraulic cylinder having a rod-end chamber and a head-end chamber. The valve arrangement includes a plurality of electrohydraulic displacement control independent metering valve modules and a return check valve disposed in an outlet between the valve arrangement and the tank to generate a back pressure for the valve arrangement. This generated back pressure may limit the rate that fluid drains from the head-end or rod-end chambers. If the drain rate is limited to the same as or less than the fill rate of the other of the head-end or rod-end chambers, voiding may be minimized. The level of the back pressure is established by a spring.
Although the electrohydraulic valve arrangement of the '059 patent may minimize voiding, it may do so inefficiently. In particular, because the back pressure restriction is always active, regardless of the likelihood of voiding, the pump supplying pressurized fluid to the electrohydraulic valve arrangement must be continuously operated at a high power usage level to overcome the continuous back pressure restriction. In addition, because the back pressure restriction is constant, velocity control of the hydraulic actuators may be limited. There may be situations when it is desirable to reduce or increase the back pressure restriction to allow for increased or decreased velocity of the associated linkage system.
The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above.